Gas Hydrates.

Prof. Attilio Citterio

Dipartimento CMIC “Giulio Natta”

http://iscamapweb.chem.polimi.it/citterio/education/course-topics/

School of Industrial and Information Engineering

Course 096125 (095857)

Introduction to Green and Sustainable Chemistry

Attilio Citterio

Overview.

• Hydrates – What they are?

• Clathrates and Supramolecular Chemistry

• Structure and Stoichiometry of gas hydrates

• Example: methane hydrate

• Other gas hydrates

• Occurrence and distribution

• Uses

Attilio Citterio

Gas Hydrates: Examples of Clathrates.

Gas hydrates: They are solids formed from hydrocarbon gas and liquid

water. They resemble wet snow and can exist at temperatures above

the freezing point of water.

They belong to a form of solid

complexes known as clathrates

inclusion compounds, existing

at low T and high P.

The supramolecular assembly is

made by two parts:

1) Host molecules of water

arranged in rigid cages

2) Mobile guest molecules (gas)

of appropriate dimensions

gas molecule

water

molecules

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Supramolecular Chemistry.

Supramolecular Connections :

• wide aggregates of molecules

• Weak non covalent

interactions

• Interactions by association

Auto organization

Auto assembling

Molecular Supramolecular

Connection by

Covalent Bonds

Interaction via

intermolecular

bonds

Attilio Citterio

Structure of Gas Hydrates.

Ice like crystalline substances made up of two or more

components

Host molecule - forms an expanded framework with void spaces

Guest component(s) - fill the void spaces

Van der Waals forces hold the lattice together

• Natural gas hydrates

• Host - water

• Guest - one or more gases

• pure methane hydrates REQUIRE -

4-6oC, 50 Atm (500 m), AND correct

concentration of gas

• Guest gases in marine seds. - methane, ethane, propane, butane, carbon dioxide, hydrogen sulfide Methane I Hydrate

ideal formula is X(gas):5.75 water

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Rules for Solid Water.

According to some authors

the crystal structure of ice

follows these two rules:

1. tetrahedral O atoms

surrounded by H atoms

2. H atoms insert between

two adjacent O atoms.

These geometrical constrains

allow to produce several

different network.

Normal Ice

structure

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Nomenclature.

Edges – hydrogen bonds

Vertices – oxygen atoms

Nomenclature Xn

X: Number of edges of side surfaces

n: Number of sides with X edges

in the cage

512 2 pentagonal dodecahedra (12 sides)

51262 6 tetrakaidecahedra (14 sides)

- methane and ethane can be accommodated - nothing bigger

- ideal formula is X(gas):5.75 water

- only one third of spaces need to be filled

- rarely are voids - 100% filled

- non-stoichiometric structures

512 68

vertices

edges

side surface

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Structure of Gas Hydrates (2).

• Three non stoichiometric

cage structures

• S-H host two molecules

Double hydrates

cubic

hexagonal

2/cell

512 51262 (T)

51264 (H)512

512 51268 (E)435663 (D)

sI

sII

sH

6/cell

16/cell 8/cell

3/cell 2/cell 1/cell

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Structure of Gas Hydrates (3).

S-I S-HS-II

46 H2O 136 H2O34 H2O

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II Hydrate Structure.

II Hydrate

17-angstrom cell, 136 water molecules,

512 pentagonal dodecahedra (12 sided)

51268 hexakaidecahedra (16 sided)

accommodates molecules up to 4.8 and 6 Å

16 small, 8 large void spaces

X(gas):17 water molecules

ideal formula will not occur because when all 8

large spaces are filled the small ones can’t be filled

void spaces are diamond shaped - can

accommodate propane and isobutane.

II Hydrate

51268 435663

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Property of Gas Hydrates.

*http://www.netl.doe.gov/scng/hydrate/about-hydrates/chemistry.htm

I I II II H* H* H*

Cavity size small med small large small small huge

Cavity shape round oblate round round round round oblate

Cavity

Description5

125

126

25

125

126

45

124

35

66

35

126

8

Number/ unit

cell2 6 16 8 3 12 1

Average radius

(Angstrom)3.91 4.33 3.902 4.683 3.91 4.06 5.71

Rel. size of

CH4 (%)88.6 75.7 88.9 67.5 88.6

Coordination

No. 20 24 20 28 20 20 36

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Influence of Host Molecules.

3

4

5

Size (Å) Hydrate

former

Cavities

occupied

No Hydrates

Structure II

Structure II

Structure I

512 + 51262

512 + 51264

7 ⅔ H2O

5¾ H2O

CO2

Xe H2S

CH4

O2

N2

Kr

Ar

N° water

5

7

Structure II

Structure I

51264

51262

No SI or SII

Hydrates

17 H2O

7⅔ H2O

C-C3H6

C2H6

(CH2)2O

C3H8

Iso-C4H10

n-C4H10

Size (Å)

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Stoichiometry.

Ratio of occupation

dependent on Pressure

and Temperature

120

100

80

60

40

20

0

-20

0

T, °CTHF·2Xe·17H2O

THF·0.5Pr4NF·17H2O

THF·5H2O

Xe·6H2O

CH4·xH2OCH4·6H2O

THF·7H2O

THF·17H2O

i1hl

i3l

i5l

i6l

■

■

■

■

■

■■

THF·2CH4·17H2O

4000 8000 12000 15000

P, bar

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Example: Methane Hydrate.

«firing ice »

Is a potential Energy Source:

• A 1 m3 block of hydrate at normal

temps and pressures will release

~ 163 m3 of methane (if filled to 90%)

• Methane hydrate energy content of ~

6860 Mj·m-3

• Can occur at water T up to 30°C (↑P)

Cryogenic SEM images

taken at very low T to

keep the hydrate stable.

Source: L. Stern / USGS

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Methane Hydrate Sample.

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Methane Hydrate Deposits.

Deposits in permafrost regions (1.4 × 1013 to 3.4 × 1016 m3)

Submarine sediments in oceans (3.1 × 1015 to 7.6 × 1018 m3)

kerogen is the only pool of carbon greater than hydrates

natural gas in hydrates 2 times greater than total fossil fuel reserves

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Distribution of Gas Hydrate.

white dot = gas samples recoveredblack dot = hydrate inferred from seismic imagingdotted lines = hydrate-containing permafrost

http://walrus.wr.usgs.gov/globalhydrate/images/browse.jpg

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Methane Hydrate Formation.

Formation:

• Sufficient gas molecules

• Water saturated by methane

• Appropriately low

temperatures

• Appropriately high pressures

if we know where these conditions

are met, it is possible to predict

where gas hydrates will form.

Not fully understood and is

dependent on methane source.

Temperature (°C)

See depth (m)

1500

2000

1000

500

-5 0 5 15 2520100

1000 m

100 m

400 m

Gas hydrate =

Stable

Gas hydrate =

Unstable

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Phase Diagram.

Scheme: isobars

in the phase

diagram.

In green the

methane hydrate

region.

H2O CH4·?H2O CH4

M-H

LM-H

H-V

H-I

LW-I

H-LW

V-LW

Te

mp

era

ture

V Vapor

Lw Liquid water

H Hydrate

M Solid methane

LM Liquid methane

I Ice

V

LM-H

M-LM

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Hydrates can form as -

finely disseminated

crystals, nodules,

layers and mass

accumulations

Progression of smaller

to larger accumulations

Gas Hydrate Formation.

• Thermogenic gas produced at high temperatures

migrates from place of origin to the ‘HYDRATE STABILITY ZONE’

• Hydrates - most likely form at the ‘gas-liquid’ interface

• BUT - could also form from gases dissolved in the liquid phase

• Gas must migrate to the ‘ZONE’ and be presented in ‘SUPERSATURATED’

quantities

Source: Stanford Univ.TEMPERATURE (°C)

PERMAFROST

TEMPERATURE (°C)

-30 -15 0 15 30

IICE-WATER

PHASE-BOUNDARY

DE

PT

H(k

m)

SEDIMENT

a)

GEOTHERMAL

GRADIENT IN

PERMAFROST

DEPTH OF PERMAFROST

1.40

1.20

1.00

0.80

0.60

0.40

0.20

GEOTHERMAL

GRADIENT

OCEAN

WATER

HYDROTHERMAL

GRADIENT

CONCENTRAZION

OF METHANE

INSUFFICIENT PER

FOR HYDRATE

FORMATION

ICE-WATER

PHASE-BOUNDARY

OCEAN FLOOR

SEDIMENT

GEOTHERMAL

GRADIENT

-30 -15 0 15 30

DE

PT

H(k

m)

b)

1.40

1.20

1.00

0.80

0.60

0.40

0.20

Attilio Citterio

Progression of Hydrate Formation/Stabilization.

Disseminated Nodular Layered Massive DisseminatedNodularLayered

Source: Brooks (1986)

hydrate lattices stabilized OR de-stabilized by co-occurring organic and

inorganic pore water constituents

hydrate is effectively ‘freshened’ as these are ‘excluded’ from the lattice

in the presence of propane hydrates can be stable at higher T and lower P

clays can stabilize hydrates (and other solids)

rapid sedimentation rates and influenced by sediment texture

authigenic carbonate rubble formation with shallow fracturing of sediments

Hydrate

Matrix

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Inhibition of Formation.

• NaCl - “anti-freeze” - lowers the temp at which a hydrate can form

3.5% solution reduces stabilization temp by 2-3 °C

• Inclusion of heavier gases increases temp (hence off-sets the salinity

problem)

• Gas hydrates can alter the concentrations of pore waters by excluding

salts or melting (’freshens’ the pore water)

• >50% Nitrogen can increase pressure required by 30%

• butane (1 - 5.8%) is included in a hydrate when pressures are less

than 103 Atm

does not form hydrates at its own vapor pressure

does not inhibit the formation of methane hydrates

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Lab. Experiment on Methane Hydrate.

• Feeding of water and methane • Hexagonal Ice and Gas

CH4

200

bar

methane

cylinder

Compressor

(0….3000 bar)

Tank

(0…250 bar)

Sensor

Drain valve Shaft

Release valve

Isolation

Thermostatic bath

(+150°C to -40°C)

Pressure cell

(Al 7075)

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Uses of Gas Hydrates.

As energy source

• Relevant reserves of

methane

• Country mainly

interested: Siberia and

Canada

• Possible use as fuel

storage

Methane hydrate energy content of

~ 6860 MJ·m-3

Methane gas – 42.8 MJ·m-3

Liquefied natural gas 16000 MJ·m-3

Gas Hydrates; 10000

Fossil fuels; 5000

Earth (includes soil,

biota, peat and detritus);

2790

Oceans (includes dissolved organic and biota); 983

Atmosphere; 3,6

Distribution of organic carbon in Earth reservoirs

(excluded dispersed carbon in rocks and sediments,

which equals nearly 1000 times this amount.

Numbers in gigatons (1015 tons) of carbon

1 cubic meter of gas hydrate

(90% site occupied) = 163 m3 of gas

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PROBLEMS with Methane Hydrates.

hydrate dissociation upon recovery; engineering challenge

expense of long pipelines across continental slope, subject to blockage with solid hydrate

methane release into atmosphere problem for climate change (20x more potent than CO2)

fragile ecosystems surround sediment surface hydrates & seeps

dissociating methane hydrate at sediment/water interface

Environmental issues:

Increase in temperature on Earth will

induce significant release of methane

from methane hydrates.

Archer et al., 2007

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Environmental Concern on Methane Escape.

lots of CH4 escaping from melting gas hydrates

powerful positive feedback on global warming

CH4 is a powerful greenhouse gas

most likely oxidizes to CO2 before it enters the atmosphere… but still!

A detailed investigation of methane hydrate dissociation during global warming was reported (Archer et al., 2007)

Westbrook et al., 2009

3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.00

Potential temperature (°C)

50

100

150

200

250

Dep

th (

m)

250 m

0 10 20 30 40 50

Methane (nM)

Upper limit of weak

part of acoustic image

of plume intersected

by the ctd cast.

Upper limit of strong

part of acoustic image

of plume intersected

by the ctd cast.

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Recovery of Methane with Sinking of CO2.

Park et al., PNAS, 2006

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Large, expensive pilot programs

focus on drilling in frozen

permafrost areas

Ex: Mallik, Canada

http://energy.usgs.gov/other/gashydrates/mallik.html

Extractive Activities.

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Uses of Hydrogen Clathrates

as Energy Storage and Carriers.

Hydrogen Storage Potential

• Binary inclusions of gas hydrate and THF

• Cage occupation

(2 H2)2·(4 H2)x·THF(1-x)17H2O

• Capacity: 4 % by weight

THF = Source: Nature 434, 743-746, 2005

Attilio Citterio

References.

1. E. Dendy Sloan, Jr., Nature, 426, 353-363, 2003

2. H. Lee, J.-w. Lee, D. Y.Kim, J. Park, Nature 434, 743-746, 2005

3. Dr. Jörg Bialas, IFM-GEOMAR, Submarine Gashydratlagerstätten

als Deponie für die CO2-Sequestrierung, 2007

4. Gabitto, J. F. and Tsouris C. “Physical Properties of Gas Hydrates: A

Review.” J. of Thermodynamics, Hindawi Pub., Vol. 2010, ID271291.

5. ] J. Caroll. Natural gas hydrates. A Guide for Engineers. Gulf

Professional Publishing, ISBN 0-7506-7569-1, 2003

6. E. D. Sloan and C. A. Koh. Clathrate hydrates of natural gases. Third

edition, CRC Press, 6000 Broken Sound Parkway NW, Suite 300, Boca

Raton, FL 33487-2742, U.S.A., 2008

7. www.ifm-geomar.de

8. http://www.imc.tuwien.ac.at/download/supramolekular_01.pdf

9. http://www.sfv.de/lokal/mails/wvf/methanhy.htm

10. http://www.cup.uni-muenchen.de/ac/kluefers/homepage/L_ac1.html